Where the Wild Things Are: An Expedition through the Cerebellopontine Angle

Introduction: Apocryphally referred to by Harvey Cushing as “The dark corner,” the cerebellopontine angle (CPA) is among the most formidable locations in neurosurgery, due to the intricate cerebrovascular and cranial nerve anatomy, deep and narrow working corridors, and close proximity of the brainstem. Although the overwhelming majority of CPA lesions are vestibular schwannoma (VS) or meningioma, it is also home to perhaps the greatest diversity of intracranial pathologies, many of which present considerable diagnostic challenges preoperatively. Our goal in this study was to survey the breadth of CPA pathology in a 16-year single-surgeon skull base practice at a large tertiary referral center, and present critical lessons derived from our approach to diagnosis and preoperative planning.

Methods: Retrospective case series.

Results: During the study period July 2002 to July2018, approximately 1,436 skull base cases were completed by the senior author (M.J.L.), of which 924 were coded as cerebellopontine angle mass lesions. Schwannomas (n = 632) and meningiomas (n = 134) accounted for the great majority of lesions (82%), followed by epidermoid/cholesteatoma (n = 33), chondrosarcoma (n = 15), and glomus/paraganglioma (n = 13), exophytic glial neoplasm (n = 12). Diseases with ten or fewer operative cases each included hemangioblastoma, ependymoma, exophytic or true CPA metastasis, neurofibroma, endolymphatic sac tumor, AVM, chordoma, solitary fibrous tumor/hemangiopericytoma, dural arteriovenous fistula (dAVF), gliosarcoma, primary squamous cell carcinoma, craniopharyngioma, malignant peripheral nerve sheath tumor (MPNST), lymphoma, aneurysm, sarcoid granuloma, glioneuronal heterotopia, primary facial nerve glioblastoma, EBV smooth muscle tumor, EBV lymphoproliferative disorder, undifferentiated small cell sarcoma, granulomatous angiitis, and CAPNON. Approaches included standard retrosigmoid in 537 operations (58%), translabyrinthine in 229 (25%), posterior petrosectomy in 82 (9%), and anterior petrosectomy/middle fossa in 76 (8%). Notable cases of discord between preoperative and postoperative diagnoses included MALT lymphoma appearing as an en plaque meningioma, gastric mucinous adenocarcinoma as a cystic schwannoma, glioneuronal heterotopia as an IAC nerve sheath tumor, glioblastoma as a schwannoma/MPNST, AVM as a dAVF, and both primary and metastatic exophytic tumors as meningioma or schwannoma.

Conclusion: Although dominated by a few common pathologies, the CPA is home to an incredible diversity of skull base lesions. The retrosigmoid craniotomy is a reliable and versatile approach, which we recommend as the operation of choice for most lesions. In patients without useful hearing, or lesions that project anteriorly into the interpeduncular cistern, the translabyrinthine is an excellent alternative, while the anterior petrosectomy is useful in more limited CPA lesions with a significant middle fossa component, and the posterior petrosectomy provides a wide corridor from the CPA to the midline and may allow for hearing preservation. For all CPA lesions, we recommend thoughtful consideration of a broad differential diagnosis, use of adjunctive imaging or laboratory studies in equivocal circumstances, and a universal preparedness for the unexpected within the OR.